This invention relates to measurement apparatus and methods, and more particularly to apparatus and methods for measuring permeability of cigarette papers in terms of CORESTA units.
The papermaking art long has appreciated the need for monitoring the degree to which a paper product is permeable to air. For relatively impermeable products, void or hole detectors which pneumatically sense the presence of such imperfections and automatically deactivate the manufacturing apparatus until the problem can be solved are sufficient. A more accurate apparatus was disclosed by Ziegenhagen in U.S. Pat. No. 3,466,925. In this apparatus, a constant vacuum source is connected to a sensing head in contact with the moving paper web. A transducer in the vacuum line senses differentials in pressure, and feeds a signal to a strip recorder, where a capacitor averages the signal. Because changes in permeability cause changes in the flow rate of air in the vacuum line, and thus changes in pressure, the average signal recorded is roughly proportional to the permeability.
Paper permeability has become increasingly important to the cigarette industry with the advent of air dilution as a primary means of controlling particulate delivery. In comparison to the pre-existing situation, cigarette manufacturers now need to measure permeability over much wider ranges than before, and they need to conduct such measurements with greater accuracy in order to insure uniformity of the end product. In order to standardize measurement techniques, the cigarette industry has adopted a standard unit of measurement, called the CORESTA unit, based upon the volume of air passing through a unit area at a predetermined pressure per unit of time. One CORESTA unit is defined as one cubic centimeter of air passing through one square centimeter of paper, driven by a pressure of one centibar.
This unit of measure is applied to all the various types of paper used in the cigarette industry. Principally, these papers include highly porous plug wrap used in the formation of filter plugs, tobacco rod wrapper of various levels of permeability, and tipping paper. The latter, which is the paper used to wrap the filter plug and join it to the tobacco rod, presents a particular problem, when used in the form of ventilated tipping paper having minute perforations to provide air dilution. Such perforations are often microscopic in size, formed by mechanical, electrostatic, or laser perforating devices. The perforated region of the tipping paper is highly porous, but variations in the performance of the perforating apparatus will cause the permeability to vary greatly. For example, assume a tipping paper perforating apparatus running at normal production speed of 500 feet per minute. If this apparatus malfunctions so that a five-foot strip of tipping paper receives no perforations whatsoever, an on-line monitoring device would sense an instantaneous change in permeabilty from a level on the order of several thousand CORESTA units to zero CORESTA and back again in less than a second. Thus, any successful online monitoring device must be capable of dealing with wide, rapid swings in permeability.
This problem is further compounded by the discovery that the relationship between airflow through a paper web and the pressure differential across the web is nonlinear. Graham, in U.S. Pat. No. 4,198,853, which disclosure is hereby incorporated by reference herein, studied the phenomenon of airflow through a paper web and found that V=A(PD).sup.N, where V is the volumetric flow rate, and PD is the pressure differential across the web, A and N are constants for each paper but vary from paper to paper depending on the permeability. From this fact, it follows that adaptations of the Ziegenhagen device will be inherently inaccurate, especially over wide ranges of permeabilty. That device depends upon an assumed linear relation between flow and pressure. In other words, Ziegenhagen assumes that a given change in flow always results in the same change in pressure; as Graham shows, this supposition simply is not true. If permeability variations are sufficiently small that the relation approximates linearity, or if accuracy requirements are not high, the device may still serve. In the cigarette industry, however, paper permeability will vary greatly, and a high degree of accuracy must be maintained.
One solution to this problem would be to maintain either flow rate or pressure differential constant, allowing direct measurement of the other variable. Both Molins, U.S. Pat. No. 3,720,095 and Stulz, U.S. Pat. No. 4,246,775 disclose on-line permeability monitoring devices wherein the pressure drop (in the former) or the flow rate (in the latter) are set at constant values. Such devices, however, inherently are incapable of dealing with wide, rapid variation in permeability. The Stultz device, for example, calls for a constant flow rate, set with a metering valve. A transducer then monitors changes in pressure, which are taken to be directly related to changes in permeability. In practice though, the flow rate will not remain constant. Faced with a sudden, wide fluctuation in permeability, the flow rate must react, the metering valve notwithstanding. When the flow rate varies, Graham teaches that changes in pressure no longer bear a linear relation to changes in permeability, rendering the Stultz device inherently inaccurate. Exactly the same reasoning applies to the Molins disclosure.
Accurate measurement in CORESTA units is possible, but only by sacrificing on-line capability. Graham discloses a method for testing a paper web in terms of CORESTA units, but this method requires iterative computer analysis of data obtained by first varying pressure while measuring flow rate, and then varying flow rate while measuring pressure. Computer analysis defines the resulting flow rate/pressure curve. Another method is disclosed in U.S. Pat. No. 4,198,854, to Washington. This disclosure teaches that one may measure the CORESTA permeabilty of a paper web by directing a plurality of medium flows at different volumetric flow rates through the web, measuring changes in pressure. Again, computer analysis of the resulting data enables one to calculate the permeability. Obviously, iterative methods such as these inherently are incapable of providing instantaneous monitoring of a paper web moving at 500 feet per minute.
A disclosure by Brown, U.S. Pat. No. 4,253,010, is a reference-type permeabilty detector and control system. A vacuum source, regulated for constant flow, is connected to sensing means at two points, a reference point and a sample point. A U-shaped manometer tube is located between these vacuum lines, so that when the sensed permeabilty at the two points is equal, the vacuum line pressures are equal, causing the fluid in each leg of the U to be at the same level. Differences in permeability, and hence pressure, cause one leg or the other of the U to be at a higher level than the other, which condition is sensed by photoelectric means. This disclosure is incapable of solving the problem of accurate, direct readout of CORESTA permeabilty. First, this is a reference-type monitor, not adapted to provide output in terms of the CORESA permeability. Second, this disclosure shares with the patents discussed above the inherent inaccuracies of a constant-flow regulated system. Finally, detection of pressure differentials by means of a fluid manometer cannot respond to rapid variations in pressure. The mass of the fluid possesses sufficient inertia to damp out rapid variations, allowing measurement only of average, rather than instantaneous, values.
Thus, one who seeks direct, accurate, on-line measure of CORESTA permeability looks in vain to the prior art. One can achieve accurate measurement, or one can achieve on-line measurement; one cannot do both.